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The textile industry generates a large amount of byproducts that must be treated before being recycled or disposed of. The treatments to extract the dyeing agents are mandatory, and involve costs and interaction with toxic reagents. A relevant amount of such byproducts are short cotton dyed fibers. Cotton fibers are high-quality cellulosic fibers and can be used as composite reinforcement. In this paper, dyed cotton fibers were used to formulate, obtain and tensile test composite materials. The impact of the presence of dyes was studied and such dyes enhanced the interphase between the matrix and the reinforcement. On the other hand, when a coupling agent was incorporated to the formulation of the composites, the dyes hindered the chemical interactions between the maleic acid and the OH groups of the cellulosic fibers. Nonetheless, the composite materials showed competitive mechanical properties that were better than other natural fiber-reinforced composites and comparable to some glass fiber-based ones. Dyed cotton fibers can be used as reinforcement without further treatment, increasing the value chain of the textile industry and decreasing the chemical treatments necessary to recycle or dispose of dyed textile fibers.

Two of the drawbacks of using natural-based composites in industrial applications are thermal instability and water uptake capacity. In this work, mechanical wood pulp was used to reinforce polypropylene at a level of 20 to 50 wt. %. Composites were mixed by means of a Brabender internal mixer for both non-coupled and coupled formulations. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to determine the thermal properties of the composites. The water uptake behavior was evaluated by immersion of the composites in water until an equilibrium state was reached. Results of water absorption tests revealed that the amount of water absorption was clearly dependent upon the fiber content. The coupled composites showed lower water absorption compared to the uncoupled composites. The incorporation of mechanical wood pulp into the polypropylene matrix produced a clear nucleating effect by increasing the crystallinity degree of the polymer and also increasing the temperature of polymer degradation. The maximum degradation temperature for stone ground wood pulp-reinforced composites was in the range of 330 to 345 ºC.